During the second year of this grant the focus will be in the following areas: 1. Structural studies on the fourth component (C4) of the complement system with special emphasis on the cleavage product C4a. 2. Structural studies on a serum precursor of C4 (Pro-C4) and the mechanism of conversion to C4. 3. Analysis of the mechanism of action of the C4b binding protein (C4-bp) recently identified by us. 4. The mechanism of activation of complement by AgA-antigen complexes using mouse myeloma proteins with antigenic specificity to phosphorylcholine and DNP.